Sleep disordered breathing is a common disorder. There are two categories of respiratory disturbances during sleep, both of which result from narrowing of the pharyngeal airway. The first is sleep apnea which is manifested by transient respiratory disturbances such as apnea or hypopnea. The second is high upper airway resistance which, by contrast, is a sustained respiratory disturbance that causes obstructive hypoventilation. In high upper airway resistance, a segment of the pharynx displays choke-point behavior, that is, progressive narrowing during inspiration that produces inspiratory flow limitation. As inspiration progresses, the pharynx narrows progressively, and the resulting inspiratory airflow is reduced and limited, such that progressive decreases in downstream pressure (superglottic pressure) fail to produce commensurate increases in airflow. High upper airway resistance occurs in episodes consisting of a series of breaths, the majority of which display inspiratory flow limitation. During these episodes of high upper airway resistance, the arterial O2 may fall and the arterial CO2 rise.
Sleep induced alveolar hypoventilation can be caused by high upper airway resistance. This is commonly seen in children and is probably the most common presentation of sleep disordered breathing in that age group. The disorder also occurs commonly in adults and produces symptoms comparable to sleep apnea, such as, sleepiness, fatigue, depression, cognitive impairment, impotence, muscle pains and headaches. Another feature of high upper airway resistance is that duration of inspiration may be prolonged compared to the duration of expiration. Furthermore, the intra-thoracic pressure may become quite negative. These two changes imply that the respiratory muscles develop substantial force and that the heart is exposed to large sub-atmospheric pressures which dilate it and act as a pre-load and after-load on the heart. These large sustained pressures can have adverse effects on cardiac rhythm and cardiac function. In addition the existence of obstructive hypoventilation can lead to respiratory failure with an increase in arterial CO2 and consequent cerebral vascular alterations leading to prominent headaches.
During inspiratory flow limitations caused by pharyngeal narrowing, the airflow is observed to be either constant or decreasing. A highly compliant tube that displays flow limitation, for example, a Starling resistor will display constant flow with a constant upstream pressure and a progressively declining downstream pressure. However, in some cases in humans and other animals, progressive decreasing downstream pressure actually causes a reduction in airflow, referred to as negative effort dependence or negative pressure dependence. In this pattern, airflow quickly rises to a peak at the beginning of inspiration and then declines subsequently during inspiration. Such negative effort dependence is thought to reflect changes in the mechanics of the constricting pharyngeal segment caused by distortion of more downstream segment or segments which, in turn, alters the mechanical behavior of the upstream segment through mechanical interdependence of the pharyngeal wall.
Traditional approaches to detection of high upper airway resistance are based on the idea that above a certain limit, the upper airway restricts airflow to a constant rate, independent of the driving pressure like a Starling resistor. Therefore, the traditional approaches to detection of resisted breathing are based on quantifying the flatness of the breaths. Various mathematical approaches are used to generate measures of flatness with the greater amount of flatness indicating a greater amount of resisted breathing. However, the inspiratory portions of resisted breaths are not exactly flat, meaning that relying on traditional methods can cause problems when detecting upper airway resistance.